Internal combustion engine ignition coil

ABSTRACT

An independent ignition type ignition coil for an internal combustion engine, comprising, arranged concentrically in a coil case ( 6 ) and with the innermost one mentioned first, a center core ( 1 ), a secondary coil ( 3 ) wound on a secondary bobbin ( 2 ) and a primary coil ( 5 ) wound on a primary bobbin ( 4 ), with epoxy resin ( 8 ) and soft epoxy ( 17 ) filled between these component members, wherein a coating is formed on the outer surface of the primary coil ( 5 ) so as to facilitate the separation from the epoxy resin ( 8 ) and the presence of this separation portion ( 50 ) between the primary coil ( 5 ) and the resin and in an interlayer of the primary coil ( 5 ) can reduce from a thermal stress produced within the secondary bobbin ( 2 ) a stress which is produced in the secondary bobbin by a heat contraction difference between the primary coil ( 5 ) and the secondary bobbin, thereby reducing the thermal stress of the secondary bobbin of the independent type ignition coil exposed to a rigorous temperature environment to prevent bobbin cracking and ensure an integrity of electrical insulation.

TECHNICAL FIELD

The present invention relates to an independent ignition type ignitioncoil for an internal combustion engine which is installed in a plug holeof the engine to be directly coupled with each spark ignition plug.

BACKGROUND ART

Since such independent ignition type ignition coil is introduced atleast a part of a coil portion within a plug hole and installed therein,a center core (in which a plurality of silicon steel plates are stackedon a magnetic path iron core), a primary coil and a secondary core arehoused with a thin cylindrical coil casing. A high voltage necessary forspark ignition is generated in the secondary coil by controlling supplyand block of a current for the primary coil. These coils are normallywound on respective bobbins and are arranged around the center core incoaxial fashion.

As the ignition coil of this kind, there are a so-called outer secondarycoil structure, in which the primary coil is arranged inside and thesecondary coil is arranged outside, and a so-called inner secondary coilstructure, in which the secondary coil is arranged inside and theprimary coil is arranged outside. Amongst, the latter is considered tobe advantageous in comparison with the former in view point of outputcharacteristics for shorter overall length of the secondary coil incomparison with the former and smaller electrostatic stray capacitance.

Namely, a secondary voltage output and a rising characteristics thereofare affected by the electrostatic stray capacitance to lower the outputand to delay rising at greater electrostatic. Accordingly, the secondarycoil having smaller electrostatic stray capacitance is considered to bemore suitable for down-sizing and higher output.

Within the coil casing housing the primary and secondary coils,insulation ability of the coils is assured by filling an insulativeresin (filled and cured).

However, when an epoxy resin is filled (filling and curing) between thecomponents of the ignition coil assembly, since curing temperature ofthe epoxy resin is typically higher than or equal to 100° C., and atnormal temperature, the insulative resin and bobbin material are exertedthermal stress due to different of linear expansion coefficients of thecomponents (difference of linear expansion coefficients of bobbin, coil,center core and the insulative resin). Thus, it becomes necessary toprovide a measure for preventing crack and interfacial delaminationbetween the materials due to thermal stress.

In Japanese Patent Application Laid-open No. Heisei 11-111545, there hasbeen disclosed an ignition coil of inner secondary coil structure, inwhich the insulative resin is filled (filled and cured) within a coilcasing housing therein the primary and secondary coils. On the otherhand, there has been disclosed that even if the resin insulationmaterial penetrates between the wire of the primary coil, sliding may becaused the wire of the primary coil and the resin insulation material bycoating wire of the primary coil by a material which is difficult to bebonded by the insulative resin to be filled.

However, in the prior art, when the primary coil and the insulativeresin are tightly fitted, the surface of the primary coil can bescratched by the insulative resin to cause peeling off of the coating.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to make an ignition coilassembly of this kind high quality and high reliability by reducing athermal stress due to a difference of linear expansion coefficients(difference of linear expansion coefficients of bobbin, coil, centercore and insulative resin) between components without causing break downof electrical insulation of a primary coil.

In order to accomplish the above-mentioned object,

(1) Namely, the first invention is that in an independent ignition typeignition coil for an internal combustion engine to be used by directlyconnecting with each ignition plug of the internal combustion engine, inwhich a center core, a secondary coil wound around a secondary bobbin,and a primary coil wound around a primary bobbin are coaxially arrangedin sequential order from inside within a coil casing, and insulativeresin being filled between these components,

a gap portion for reducing stress generated within the secondary bobbinby thermal shrinkage difference of the first coil and the secondarybobbin among thermal stress created within the secondary bobbin, isprovided together with the insulative resin between the primary bobbinand the primary coil and between the layers of the primary coil.

This gap is at least one delaminated portion formed between “theinsulative resin (for example, epoxy resin) filled between the primarybobbin and the primary coil” and the “primary coil”, between the“insulative resin filled between the primary bobbin and the primarycoil” and the “primary coil”, and between the “primary coil” and“insulative resin filled between layers of the primary coil”.

As more particular mode of implementation, the primary coil is providedwith the coating film or coating which is easy to delaminate between theprimary coil and the insulative resin filled around the primary coil, isprovided with the coating film or coating easy to delaminate between thebobbin surface and the insulative resin contacting on the bobbinsurface, and is provided, in place with the coating film or coating,with an insulative sheet having low bonding ability with epoxy.

As these coating film or coating, overcoating containing material havingsmall friction coefficient, such as nylon, polyethylene, Teflon or thelike and material having small bonding ability with epoxy resin is used.

After curing epoxy, when temperature is lowered, delamination is causedin the portion having small tension stress at the interface betweenepoxy and the primary coil or the primary bobbin and small bondingability with epoxy, due to difference of linear expansion coefficientsof copper and epoxy.

As an effect of the present invention, when thermal shrinkage is causedin the ignition plug by lowering of temperature after stopping operationof the engine, relative expansion force in circumferential directionacts on the secondary bobbin by thermal shrinkage difference (linearexpansion coefficient difference). On the other hand, from the primarycoil and the secondary coil, tension force acts on the secondary coilrelatively in circumferential direction via the insulativeresin. Bymultiplier effect of these, large internal stress σ is created in thesecondary bobbin. In the present invention, by interposing the gap (forexample, the foregoing delaminated portion) between the primary bobbinand the primary coil and/or between the layers of the primary coil, itbecomes possible to block transmission path of the tension force in thecircumferential direction otherwise acting on the secondary bobbin fromthe primary coil.

Accordingly, among the stress σ created within the secondary bobbin, byreducing the stress component σ1 created within the secondary bobbin bythermal shrinkage difference of the primary bobbin and the secondarybobbin, total internal stress σ can be significantly reduced (weaken).By examples of CAE (Computer Aided Engineering) analysis made by theinventors, by reducing the foregoing stress component σ1, at least 20%of the total internal stress can be reduced. The reduction value of theinternal stress was confirmed in connection with the ignition coilinserted into the plug hole of the internal combustion engine to bedirectly connected to the ignition plug, and the outer diameter of theinserted portion is ø18 to ø27 mm (the thin cylindrical type ignitioncoil of this size typically has 0.5 to 1.2 mm of thickness of theprimary bobbin, 0.7 to 1.6 mm of thickness of the secondary bobbin, and50 to 150 mm of bobbin length).

Even when the foregoing gap (for example, laminated portion) is providedbetween the primary bobbin and the primary coil and/or between thelayers of the primary coil, since the primary coil is low potential(substantially ground potential), concentration of electric fieldbetween the primary coil will never be caused. Also, by tightly fittingthe secondary coil, the insulative resin and the primary bobbin withoutgap, insulation between the primary coil and the secondary coil can besufficiently assured. It has also been confirmed by the result of testthat concentration of electric field by line voltage of the secondarycoil can be satisfactorily prevented. Thus, insulation break down can beprevented.

(2) Furthermore, in addition to the foregoing first invention, whenmodified PPE (modified polyphenylene ether) is used for the secondarybobbin, the internal stress σ can be further reduced in viewpoint ofimprovement of material of the secondary bobbin by containing inorganicfiller (glass fiber, Mica, Talk or the like) in the content of greaterthan or equal to 20% in the secondary bobbin.

Modified PPE is superior in bonding ability with epoxy resin serving asthe insulative resin, and has good molding ability and insulationability. Therefore, it can contribute for quality stability of thesecondary bobbin. When the inorganic filler content is less than 20%,the difference of linear expansion coefficients with other component(center core, primary coil, secondary coil or the like) becomes large tomake the internal stress (thermal stress) σ large, For example,according to the example of CAE analysis, if the foregoing σ1 is notreduced, the internal stress σ created in the secondary bobbin becomesas large as about 90 to 100 MPa upon occurrence of abrupt temperaturedrop if the ignition coil is placed in temperature environment varyingfrom 130° C. to −40° C.

In contrast to this, according to the present invention, the internalstress σ can be lowered to be less than or equal to 70 MPa tosuccessfully prevent longitudinal cracking of the secondary bobbin. Itshould be noted that as optimal example of lowering of the internalstress σ with maintaining bolding ability (flowability of the resin) ofthe secondary bobbin, it is proposed a material containing 45 to 60 Wt %of modified PPE, 15 to 25 Wt % of glass fiber, 15 to 35 Wt % ofnon-fibric inorganic filler. The detail will be discussed in thediscussion of the embodiment.

Furthermore, in viewpoint of the linear expansion coefficient loweringthe foregoing internal stress σ, particularly, when resin flow directionin resin molding is axial direction of the bobbin, the linear expansioncoefficient in the direction perpendicular to the resin flow direction(it becomes important point for preventing longitudinal cracking of thebobbin to suppress internal stress in the direction corresponding toradial direction and circumferential direction of the bobbin,particularly in circumferential direction) of 35 to 75×10⁻⁶ in averageat −30° C. to −10° C. in test method according to ASTM D 696. Detail ofthis will be discussed in the discussion of the embodiment.

As more particular embodiment, by forming the coating film or coat layeron the outermost layer of the primary coil containing component havingno affinity or causing no chemical reaction with the insulative resin(for example, epoxy resin), delamination is caused between the primarycoil and the insulative resin to form the gap portion. The componenthaving no affinity or causing no chemical reaction with the insulativeresin is the material expressed by CH₂CH₂n(n≧2) orCH₂—CH(CH₃)n(n≧2), for example, nylon, polyorefin such aspolyethylene, polypropylene or the like, fruorinated resin, fluorinatedester, fluorinated rubber, wax, fatty acid ester.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal section of one embodiment of an ignition coilfor an internal combustion engine according to the present invention;

FIG. 2 is an enlarged illustration showing a portion B of FIG. 1 in acondition enlarged and transversely oriented;

FIG. 3 is a cross section taken along line A-A′ of FIG. 1;

FIG. 4 is an enlarged section of portion C FIG. 2;

FIG. 5 is an enlarged section of portion C of another embodiment of thepresent invention;

FIG. 6 is a top plan view of an igniter casing of the foregoingembodiment;

FIG. 7(a) is a front elevation showing an ignition driver circuit to beused in the foregoing embodiment, to be transfer molded, (b) is its topplan view, and (c) is a top plan view showing a condition where theignition driver circuit is mounted and before transfer molding;

FIG. 8 is a diagrammatic illustration showing a mode of insulation breakdown in the case where crack is caused in respective portion of theignition coil;

FIG. 9 is a section of the primary coil to be employed in the foregoingembodiment;

FIG. 10 is a diagrammatic illustration showing a condition where a partof a secondary bobbin to be employed in the foregoing embodiment is cutinto half to be locally sectioned;

FIG. 11 is an enlarged section of a portion P of FIG. 10;

FIG. 12 is a diagrammatic illustration showing a relationship between alinear expansion coefficient in peripheral condition (perpendiculardirection relative to flow direction in resin molding) of a secondarybobbin and a stress generated in the secondary bobbin;

FIG. 13 is a diagrammatic illustration showing a relationship between acontent of Mica in the secondary bobbin and the linear expansioncoefficient;

FIG. 14 is a diagrammatic illustration showing a stress generated in thesecondary bobbin and number of heat cycle;

FIG. 15 is a longitudinal section of another embodiment of the ignitioncoil for the internal combustion engine according to the presentinvention and an enlarged section of a portion E; and

FIG. 16 is an enlarged section of a portion D of FIG. 9.

BEST MODE FOR IMPLEMENTING THE INVENTION

Embodiments of the present invention will be discussed with reference tothe drawings.

FIG. 1 is a longitudinal section of one embodiment of an ignition coilfor an internal combustion engine according to the present invention,FIG. 2 is an enlarged illustration showing a portion B of FIG. 1 in acondition enlarged and transversely oriented, and FIG. 3 is a crosssection taken along line A-A′ of FIG. 1.

Within a thin cylindrical casing (outer casing) 6, a center core 1, asecondary coil 3 wound around a secondary bobbin 2 and a primary coil 5wound around a primary bobbin 4 are arranged in coaxial fashion fromcenter (inside) to outside. On outside of the outer casing 6, a sidecore forming a magnetic path with the center core 1 is mounted.

The center core 1 is formed by stacking a large number of silicon steelplates or directional silicon steel plates set widths into severalstages, by press for increasing sectional area, as shown in FIG. 3, forexample. On both ends in axial direction of the center core 1, magnets 9and 10 are arranged adjacent the center core 1. The magnets 9 and 10 areadapted to operate the ignition coil lower than or equal to saturationpoint of magnetization curve of the core by generating a magnetic fluxin opposite direction to the magnetic flux of the coil passing throughthe center core 1. The magnet may also be arranged only at one end ofthe center core 1. The reference numeral 24 denotes an elastic body (forexample, rubber) absorbing thermal expansion in axial direction of thecenter core 1.

As shown in FIG. 2, between the center core 1 inserted within thesecondary bobbin 2 and the secondary bobbin 2, so-called soft epoxyresin (flexible epoxy) 17 is filled, and within gaps between respectivecomponents of the secondary bobbin 2, the secondary coil, the primarybobbin 4, the primary coil 5 and the outer casing 6, hard epoxy resin(thermosetting epoxy resin) is filled.

The soft epoxy resin 17 is epoxy resin having soft property (elastmer)having glass transition point lower than or equal to a normaltemperature (20° C.) and having elasticity at a temperature higher thanor equal to the glass transition point, and can be a mixture of epoxyresin and modified fatty series polyamine (thermosetting epoxy resin) 8,for example.

The reason why soft epoxy resin 17 is selected as the insulative resinbetween the center core 1 and the secondary bobbin 2, is that so-calledpencil coil (independent ignition type ignition coil of the type to beinserted into a plug hole) is faced to severe temperature environment(thermal stress in a temperature range between about −40° C. to 130°C.), and in addition thereto, since the difference between the linearexpansion coefficient of the center core 1 (13×10⁻⁶) of the center core1 and the linear expansion coefficient of the hard epoxy resin of thehard epoxy region (40×1⁻⁶) is large, if ordinary epoxy resin (epoxyresin composition harder than soft epoxy resin 17) is used, crack may beformed in the epoxy resin by a heat shock to cause insulation breakdown. Namely, as a measure for the heat shock, the soft epoxy resin 17which is an elastic body superior for absorbing heat shock and hasinsulation ability, is used.

Here, discussion will be given for the secondary bobbin 2. The shownembodiment of the secondary bobbin 2 is based on the following finding.

{circle around (1)} The secondary bobbin 2 satisfies a condition of[allowable stress of secondary bobbin σ0> (stress to be generated at(−40 C. −glass transition point Tg of soft epoxy resin 17) σ]. Here, asone example, the secondary bobbin having the soft epoxy resin which hasglass transition point Tg at −25° C., will be discussed as example.

For example, when the glass transition point Tg of the soft epoxy resin17 is −25° C., when the secondary bobbin 2 causes shrinkage to causetemperature drop after stopping operation of the internal combustionengine as placed in the environment causing temperature variation from130° C. to −40° C., shrinkage of the secondary bobbin 2 in thetemperature range from 130° C. to −25° C. can be accommodated by elasticabsorption of the soft epoxy resin 17. Therefore, among thermal stress σto be caused within the secondary bobbin 2, a component exerted from thecenter core 1 is substantially zero. It should be appreciated that as awhole, when thermal shrinkage of the secondary bobbin 2 is caused, theprimary coil 5 and the secondary coil 3 having smaller linear expansioncoefficient (thermal expansion coefficient) than the secondary bobbin 2suppresses thermal shrinkage of the secondary coil 3 via the hard epoxyresin 8. In other words, the primary coil 5 and the secondary coil 3apply tension force in peripheral direction relative to the secondarybobbin 2. By this, a sum of thermal stress component σ1 acting from theprimary coil 5 and thermal stress component σ3 acting from the secondarycoil 3 becomes a major component of an internal stress σ of thesecondary bobbin 2.

In a temperature range from −25° C. to −40° C., the soft epoxy resin 17transits to glass state. By this, shrinkage (deformation) of thesecondary bobbin 2 is also prevented from the side of the center core 1.Therefore, a thermal stress σ3 applied by a force from the center coreside is added to thermal stresses σ1 and σ2 applied by the primary coiland the secondary coil set forth above. A stress as a sum of these σ1,σ2 and σ3 becomes a major component of the internal stress σ of thesecondary bobbin 2.

The thermal stress σ caused in the secondary bobbin 2 is expressed byσ=E·å=E·α·T. E is a Young's modulus, å is a strain,α is a linearexpansion coefficient of the secondary bobbin and T is a temperaturevariation (temperature difference). When the allowable stress σ0 of thesecondary bobbin 2 is greater than the generated stress σ (σ<σ0),breakage of the secondary bobbin 2 is not caused.

{circle around (2)} material of the secondary bobbin 2 is to be selecteda material having good bonding ability with the epoxy resin 8. Whenbonding ability with the epoxy resin 8 is low, delamination is causedbetween the secondary bobbin 2 and the epoxy resin 8 to potentiallycause insulation break-down.

Here, discussion will be given for a mechanism of insulation break-downupon occurrence of delamination (including formation of crack of theinsulative resin) between the insulative resin and the bobbin materialwith reference to FIG. 8.

FIG. 8 shows a part of the pencil coil of inner secondary coil structurein enlarged form, and is a partial enlarged section of the case where aplurality of collars (collars for setting respective spool area) 2B forseparately winding the secondary coils are arranged on the outer surfaceof the secondary bobbin 2 in spaced apart in axial direction.

Among the epoxy resin 8, the epoxy resin 8 to be filled between thesecondary bobbin 2 and the primary bobbin 4 reaches the outer surface ofthe secondary bobbin 2 as penetrated between wire of the secondary coilin addition between the secondary coil 3 and the primary bobbin 4 bylamination (vacuum pressure impregnation). On the other hand, betweenthe center core 1 and the secondary bobbin 2, the soft epoxy resin 17 isfilled as set forth above.

In this case, when contact strength (bonding strength) between theinsulative resin and the secondary and primary bobbins is low,delamination can be caused between the secondary bobbin 2 and theinsulative resin 8 penetrating into the secondary coil as shown byreference sign I, and between the collar of the secondary bobbin and theinsulative resin 8 as shown by reference sign II. On the other hand,regions between the insulative resin 8 and the primary bobbin 4 as shownby reference sign III and between the insulative resin 17 and thesecondary bobbin 2 as shown by reference sign IV are considered asregions potentially causing delamination.

When delamination is caused at the position shown by the reference signI, concentration of electric field can be caused by line voltage of thesecondary coil 3 through a delaminated portion (gap) to cause partialdischarge between wire of the secondary coil 3 to generate heatresulting in burning out of enamel coating of the wire of the secondarycoil to cause layer shorting. On the other hand, when delamination iscaused in the position shown by the reference sign II, concentration ofelectric field is caused between wire between the separately woundadjacent areas of the secondary coil 3 to cause layer shorting due topartial discharge similarly to the above. When delamination is caused inthe position shown by the reference sign III, insulation break down iscaused between the secondary coil 3 and the primary coil 5. When thedelamination is caused in the position shown by the reference sign IV,insulation break down is caused between the secondary coil 3 and thecenter core 1.

In the shown embodiment, in order to satisfy the foregoing condition,modified PPE superior in bonding ability with the epoxy resin is used asthe material of the secondary bobbin 2. This material contains inorganicsubstance (glass filler, Mica or the like) for certainly providingreinforcement. Furthermore, in the shown embodiment, in order to satisfythe foregoing condition, namely, for making the linear expansioncoefficient α of the secondary bobbin as small as possible and forrealizing the foregoing allowable stress σ0>σ, inorganic substance iscontained in a content greater than or equal to 20 Wt %, and morepreferably greater than or equal to 30%. On the other hand, in order toassure injection molding ability of the secondary bobbin 2, it isnecessary to improve fluidability of the resin in molten condition. Theinorganic substance may be not only fiber type, such as glass filler orthe like but also non-fibric inorganic substance, such as Mica or thelike.

FIG. 10 shows a sectional perspective view showing a part of thesecondary bobbin in the shown embodiment illustrated in cut into half.Flow direction of the resin upon molding of the secondary bobbin of theshown embodiment is axial direction of the bobbin, and diametricaldirection and circumferential direction of the bobbin is perpendiculardirection relative to the flow direction of the resin of the secondarybobbin. FIG. 11 is an illustration diagrammatically showing the portionP of FIG. 10 in enlarged form. Glass fiber as filler is oriented in theresin flow direction. Accordingly, linear expansion coefficient of thesecondary bobbin in the axial direction is sufficiently small incomparison with the diametrical direction and circumferential directionperpendicular to the axial direction. When the linear expansioncoefficients in the diametrical direction and circumferential directionare desired to make smaller without sacrificing flowability of theresin, it becomes necessary to make linear expansion coefficients indiametrical direction and circumferential direction by admixing anon-fibric filler material (e.g. Mica, talk or the like) in addition toglass fiber. In order to withstand to inner stress (thermal stress) σ,the secondary bobbin 2 is required to make the linear expansioncoefficient in circumferential direction of the bobbin (perpendiculardirection with respect to the resin flow direction).

FIG. 13 shows a relationship between the Mica content and linearexpansion coefficient in a direction perpendicular to the resin flowdirection (average linear expansion coefficient of −30° C. to −10° C. ina test method in accordance with ASTM D 696) in the case where thesecondary bobbin 2 is formed with modified PPE (base containing 20 Wt %of glass fiber). In FIG. 13, E-6 represents 10⁻⁶. In this case,inorganic filler is 20 Wt % in total (20 Wt % of glass fiber, 0 Wt % ofMica) and the linear expansion coefficient is about 70×10⁻⁶ (in case oftest example, 49.3×10⁻⁶). When the inorganic filler is 20 Wt % of glassfiber and 20 Wt % of Mica, the linear expansion coefficient is about50×10⁻⁶ (in case of the test example, 49.3×10⁻⁶). When the inorganicfiller is 20 Wt % of glass fiber and 30 Wt % of Mica, the linearexpansion coefficient is about 40×10⁻⁶ (in case of the test example,39.6×10⁻⁶). For example, when the linear expansion coefficient isdesired to restrict in a range of about 40 to 50×10⁻⁶, if content ofglass fiber is 20 Wt %, the content of Mica becomes 20 to 30 Wt %. Whenthe content of glass fiver is 15 to 25 Wt %, and the linear expansioncoefficient is desired to restricted in a range of about 40 to 50×10⁻⁶,required content of Mica becomes 15 to 35 Wt %. More particularly,modified PPE is 45 to 60 Wt %, glass fiber is 15 to 25 Wt % and Mica is15 to 35 Wt %. As optimal example, in the shown embodiment, thesecondary bobbin 3 contains 55 Wt % of modified PPE, 20 Wt % of glassfiber and 30 Wt % of Mica. As shown in FIG. 13, Mica content and linearexpansion coefficient in perpendicular direction is substantiallyproportional relationship.

It should be noted that the modified PPE containing 50% of inorganicsubstance has linear expansion coefficient a of 20 to 30×10⁻⁶ in theresin flow direction upon molding in the temperature range of −30° C. to100° C.

Here, for certainly attaining strength of the secondary bobbin 2, itshould be natural that greater thickness of the bobbin is better.However, since the pencil coil is typically required to be inserted intoa thin plug hole in the extent of φ19 to φ28 mm, the external diameterof the coil portion to be inserted there into should be in a extent ofφ18 to φ27 mm including the side core. Within such narrow space, epoxyresin 8 has to be filled between the components, such as the coil casing6, the primary coil 5, the primary bobbin 4, the secondary coil 3, thesecondary bobbin 2, the center core 1 and so forth and a gap defined inthe components so as to fill up the defect. Accordingly, the thicknessesof respective parts are desire to be as small as possible.

In the shown embodiment, the thickness of the primary bobbin is set at0.5 mm to 1.2 mm, the thickness of the secondary bobbin is set at 0.7 to1.6 mm, and a length of the bobbin is set at 50 to 150 mm.

The secondary coil 3 to be wound around the secondary bobbin 2 has alinear expansion coefficient of about 22×10⁻⁶ at −40° C. in a conditionwhere epoxy resin is impregnated between wire. On the other hand, theprimary coil 5 to be wound around the primary bobbin 4 has a linearexpansion coefficient of about 22×10⁶ at −40° C. in the condition whereepoxy resin is impregnated between wire. It should be noted that thelinear expansion coefficient is determined by a testing method inaccordance with ASTM D 696.

The secondary coil 3 is separately wound about 5000 to 35000 turns intotal using enamel line having line diameter of about 0.03 to 0.1 mm. Onthe other hand, the primary coil 5 is wound about 100 to 300 turns intotal over several layers (here two layers) with winding several tensturns per each layer using enamel line having line diameter of about 0.3to 1.0 mm. Outer sheath structure of the primary coil will be discussedlater.

The primary bobbin 4 is formed of PBT containing rubber. The reason whyPBT is used, is for attaining a linear expansion coefficient comparablewith or in a range ±10% of the linear expansion coefficient of epoxyresin 8. Furthermore, by containing rubber, bonding ability with epoxyresin 8 can be increased. Particularly, the composition of the materialof the primary coil is 55 Wt % of PBT, 5 Wt % of rubber, 20 Wt % ofglass fiber, 20 Wt % of plate form elastmer. It is also possible to formthe primary bobbin and the secondary bobbin with the same PPS materialto lower total cost.

For the primary coil 5, in addition to a coating 5A of insulativematerial (for example, ester imide, amide imide, urethane or the like)in a thickness of 10 to 20 μm around copper line (φ500 to 800 μm) asshown in diagrammatic illustration of FIG. 9, a coating (overcoating) 5Bto be easily separating the primary coil 5 from the insulative resin(epoxy resin) 8 filled around the primary coil, are provided. Theovercoating 5B is prepared by adding any one of nylon, polyethylene,Teflon or the like to provide sliding ability to the same material asthe insulative material 5A in a content of several %, and the thicknessthereof is 1 to 5 μm.

The reason why overcoating having not so high bonding ability to theepoxy resin 8, is to reduce stress component σ1 created in the secondarybobbin among stress σ created in the secondary bobbin by a difference ofthermal shrinkages of the primary coil 5 and the secondary coil 2(linear expansion coefficient difference) (for satisfying the foregoingcondition).

Namely, by presence of the foregoing overcoating 5B, a delaminatedportion (gap) 50 is formed between the primary coil 5 and the epoxyresin 8 presented around the primary coil 5 as shown in FIG.4. Thedelaminated portion 50 may also be formed between the epoxy resin 8filled between the primary bobbin 4 and the primary coil 5 and theprimary coil 5 or between the layers of the primary coil 5. It should benoted that FIG. 4 is an enlarged section of the portion C of FIG. 2 andis drawn based on tomogram (30 to 40 times of magnification) ofmicroscope taken on the portion corresponding to the portion C.

By interposing the gap (delaminated portion) 50 between the primarybobbin 4 and the primary coil 5 or between the layers of the primarycoil 5, it becomes possible to block a path of tension force (tensionforce based on difference of heat expansion) of the primary coil and thesecondary bobbin) in circumferential direction acting from the primarycoil 5 to the secondary bobbin. Accordingly, by reducing the stresscomponent σ1 applied by the presence of the primary coil among stress σcreated within the secondary bobbin, σ can be reduced (weaken) in theextent greater than or equal to 20%. Also, the linear expansioncoefficient of the modified PPE is improved by blending more than orequal to 20% of inorganic filler as set forth above for reducinginternal stress (thermal stress) by improvement of material of thesecondary bobbin. According to example of CAE analysis by the inventors,the stress σ generated in the circumferential direction of the secondarybobbin 2 (also in perpendicular direction relative to resin flowdirection in molding of the bobbin, hereinafter referred to as θdirection) can be significantly reduced by multiplier effect with stressweakening effect of the gap 50.

FIG. 12 shows the relationship between the linear expansion coefficientin the direction perpendicular to the resin flow direction (axialdirection of the bobbin) of the secondary bobbin and the stressgenerated in the secondary bobbin (θ direction), in the shownembodiment.

The generated stress (thermal stress) of the secondary bobbin of FIG. 12is derived as internal stress in θ direction to be created at −40° C. bygenerating three-dimensional model of the ignition coil using the CAEanalysis soft ware and inputting material property values (linearexpansion coefficient, Young's modulus, Poisson's ratio) of respectiveparts, and taking the stress generated at a temperature of 130° C. forcuring epoxy. The linear expansion coefficient in the property valueuses the material of the secondary bobbin of 35 to 75×10⁻⁶ in average at−30° C. to −10° C., as approximated value of −40° C.

In FIG. 12, the solid line A corresponds to the shown embodiment (oneprovided the foregoing delaminated portion 50 around the primary coil),in which, with taking the secondary bobbin material (gas filler 20 Wt %base of FIG. 12 with Mica content of 0 Wt %, 20 Wt % and 30 Wt %)exemplified in FIG. 13 into account, CAE analysis is performed using onehaving the linear expansion coefficient of 35 to 75×10⁻⁶ in average inthe temperature range of −30° C. to −10° C. as an approximated value ofthe linear expansion coefficient of the secondary bobbin, particularlyusing approximated linear expansion coefficient at −40° C. in θdirection of five secondary bobbins having linear expansion coefficientsof about 40×10⁻⁶ (strictly 39.6×10⁻⁶), about 50×10⁻⁶(strictly49.3×10⁻⁶), about 70×10⁻⁶ (strictly 66.8×10⁻⁶), 35×10⁻⁶ and 75×10⁻⁶ astolerance.

As a result of analysis, when the average of the linear expansioncoefficient of the secondary bobbin at approximately −40° C. (−30° C. to−10° C.) is 35 to 75×10⁻⁶ (the lower limit value 35 of the average isbased on restriction of the blending amount of the inorganic fillercapable of molding of the secondary bobbin), analysis result where thestress generated by the secondary bobbin becomes 70 MPa [allowable upperlimit of internal stress (thermal stress) of the secondary bobbin takenas target by the inventors].

While less than or equal to 70 MPa of the generated stress is based onCAE analysis by the inventors, base of the numerical value is passedheat cycle test (test repeating temperature variation in a range of 130°C. to −40° C.) sufficiently satisfying durability of the ignition coilfor the internal combustion engine of this kind. FIG. 14 is anillustration showing a characteristic test of generated stress in thesecondary coil 2 and number of heat cycles, in which a horizontal axisrepresents number of heat cycles and a vertical axis representsgenerated stress, and the range less than or equal to 70 MPa is therange where crack is not caused in the secondary bobbin 2 at 300 timesor more of the heat cycle.

The solid line B in FIG. 12 shows comparative example representative ofresult of analysis of generated stress of the secondary bobbin in thecase where the linear expansion coefficient in the θ direction is setsimilar to that of the solid line A in the ignition coil, in which theforegoing delaminated portion 50 is not provided around the primarycoil. In this case, the generated stress in the circumferentialdirection of the secondary bobbin becomes greater than or equal to 80MPa.

Even if the foregoing delaminated portion 50 us provided between theprimary bobbin 4 and the primary coil 5 and between the layers in theprimary bobbin 5, since the primary coil 5 low potential (substantiallyground potential), concentration of the electric field between theprimary coil is not caused. Furthermore, by tightly fitting thesecondary coil 3, the epoxy resin 8 and the primary bobbin 4 withoutforming gap, insulation between the primary coil and the secondary coilcan be certainly attained. In addition, it has been confirmed as aresult of test by the inventors to satisfactorily achieve prevention ofconcentration of electric field by line voltage of the secondary coil.

Particularly, in the shown embodiment, by using PBT containing rubber inthe primary bobbin 4, bonding ability with the epoxy resin 8 can beincreased and delamination of the epoxy resin 8 on the inner diameterside of the primary bobbin 4 can be certainly prevented to achieve goodinsulation performance by maintaining bonding ability between thesecondary coil 3, the epoxy resin 8 and the primary bobbin 4.

It should be noted that the primary bobbin 4 may be formed ofthermoplastic resin, such as PPS (polyphenyl sulfide), modified PPE orthe like.

For the coil casing 6, thermoplastic resin, such as PBT, PPS modifiedPPE or the like may be used. On the outer surface of the coil casing,the side cores 7 are mounted. The side core 7 is cooperated with thecenter core 1 for forming the magnetic path, and is formed by roundingthin silicon steel plate or directional silicon steel plate in thethickness of 0.3 mm to 0.5 mm into cylindrical form.

The reference numeral 20 denotes an ignition circuit unit (igniter)coupled with the upper portion of the coil casing 6. Within a unitcasing 20 a, an electronic circuit (ignition driver circuit 23) fordriving the ignition coil is housed, and a connector portion 21 forexternal connection is integrally molded with the unit casing 20 a.

The ignition driver circuit 23 in the shown embodiment is finallytransfer molded. FIG. 7(a) shows a front elevation of an independentproduct of the ignition driver circuit 23, (b) is a top plan viewthereof, and (c) shows a condition where hybrid IC 30 a and a powerelement (semiconductor chip) 30 b for the ignition driver circuit aremounted on a base (substrate) 31 with a terminal 33 before transfermolding. As shown in FIGS. 7(a) to (c), after mounting the hybrid IC 30a and the power element 30 b on the base 31, transfer molding 32 isprovided.

FIG. 6 shows a condition where the transfer molded ignition drivercircuit 23 is mounted in the unit casing 20 a. Upon mounting, afterconnecting the terminal 33 of the ignition driver circuit 23 and theconnector terminal 22 on the side of the unit casing 20 a, epoxy resin 8is filled and cured in the unit casing 8, which is illustrated in thecondition where the transfer molded ignition driver circuit 23 is seenthrough. The ignition driver circuit 23 is buried with the epoxy resin8.

In the shown embodiment, circuit elements other than a power transistoramong the ignition driver circuit 23, which are not suited forintegration into a chip, for example, noise suppressing capacitor(eliminated from illustration) is externally mounted on outside of apencil coil. The noise suppressing capacitor is arranged between a notshown power source line and ground and prevents noise generated by powersupply control of the ignition coil

By employing such transfer molded ignition driver circuit 23, theignition driver circuit 23 can be integrated into single chip toadvantageously simplified the manufacturing process to lower a cost,input current can be made small, and so on.

The reference numeral 11 denotes a high voltage diode, 12 denotes a leafspring, , 13 denotes a high voltage terminal, 14 denotes a spring forconnection with the ignition plug connection, and 15 denotes a rubberboots for connection of the ignition plug. The high voltage diode 11serves for preventing excessively advanced ignition when a high voltagegenerated in the secondary coil 3 is supplied to the ignition plug viathe leaf spring 12, the high voltage terminal 13 and the spring 14.

Major operations and effects of the shown embodiment are as follows.

(1) Even the independent ignition type ignition coil subject to severetemperature environment as installed within the plug hole, the internalstress (thermal stress) σ generated in the secondary bobbin can be madesmall.

Accordingly, with the shown embodiment, the internal stress σ of thesecondary bobbin can be significantly reduced to certainly preventcranking (longitudinal cracking prevention) of the secondary bobbin. Fortesting, temperature variation in a range of 130° C. to −40° C. isrepeatedly applied for 300 times to observe the secondary bobbin 2.Then, it has been confirmed that damage is not caused in the secondarybobbin 2 and good condition can be maintained.

(2) On the other hand, even when the gap 50 is provided as set forthabove, since bonding ability (tight fitting ability) of epoxy resin tothe secondary bobbin 2 and bonding ability of epoxy resin for inner sideof the primary bobbin 4 are high, highly reliable pencil coil can beprovided without sacrificing insulation ability.

It should be noted that, in the foregoing embodiment, the gap 50 isformed between the primary coil 4 and the insulative resin 9 therearound. However, the effect (1) of the shown embodiment set forth abovecan be expected even when the gap portion (laminated portion) 51 isformed between the insulative resin (epoxy resin) 8 filled between theprimary bobbin 4 and the primary coil 5 as shown in FIG. 5, and theprimary bobbin 5.

For example, in the embodiment shown in FIG. 5, on the bobbin surface(surface on outside of the bobbin) on the side where the primary coil iswound, among the primary bobbin 4, by applying the overcoating 4A(coating layer of coating)which easy to separate between the bobbinsurface and the epoxy resin facing the bobbin surface, formation of thegap portion 51 is assured. The material of the overcoating 4A is similarmaterial as the overcoating set forth above. On the other hand, it isalso possible to stick a sheet having small bonding force to epoxy onthe outer surface of the primary bobbin instead of the overcoating setforth above.

On the other hand, it is also possible to provide both of the foregoinggaps 50 and 51.

FIG. 15 is a partially eliminated section showing another embodiment ofthe present invention. While not illustrated, between the primary bobbin4 and the primary coil 4 and/or between the layers of the primary coil5, stress reducing gaps (delaminated portions) 50 and 51 similar to theabove are provided. On the other hand, the construction thereof issimilar to the embodiment set forth above except for the followingpoint. The same reference numerals to the foregoing embodiment identifythe same or common elements.

Namely, what is different from the embodiment set forth above, is thatinstead of filling the soft epoxy resin 17 between the center core 1 andthe secondary bobbin 2, the center core 1 is preliminarily coated withan insulating member 60, such as silicon rubber, urethane, acryl resinor the like before arranging inside of the secondary bobbin 2, in place.The coated center core is arranged within the secondary bobbin, and hardepoxy resin 8 is filled between the center core land the secondarybobbin 2.

With the shown embodiment, in addition to achievement of similar effectto the first embodiment, the following operation and effect can beachieved. By absorbing heat shock between the center core 1 and thesecondary bobbin 2 by the elastic member (center core coating) 60, itcan contribute for reducing of thermal stress σ of the secondary bobbin.Furthermore, in comparison with the operation for filling and curing thesoft epoxy resin into the narrow space between the secondary bobbin andthe center core (filling and curing under vacuum pressure), the centercore coating 60 can be done for the independently. Also,normal fillingand curing of hard epoxy resin between the center core and the secondarybobbin to be performed after insertion of the center core 1 with thecoating into the secondary bobbin, can be easily performed for lowviscosity in comparison with the soft epoxy to achieve lowering ofoperation cost. In addition, absorbing of magnetic vibration generatedfrom the center core can be efficient to reduce lowering of noise.

The shown ignition coil is constructed with a circuit shown in FIG. 5 ofJapanese Patent Application Laid-Open No. 10-325384, and operates asshown in FIG. 8 in the same publication.

In the primary coil 5, coatings 5A and 5B of insulative body (forexample ester imide, amide imide, urethane or the like) of thickness of10 to 20 μm is provided around the copper line (φ500 to 800 μm) asdiagrammatically shown in FIG. 9. In the shown embodiment, the firstcoating 5A is ester imide and the second coating 5B is amide imide toform two layer coating.

As diagrammatically shown in FIG. 16, the outer coating 5B contains acomponent 5C having no affinity or not chemically react with epoxy resin(for example, nylon, polyolefin, such as polyethylene, polypropylene,fluorinated resin, fluorinated elastmer fluorinated rubber, wax, fattyacid ester). In the shown embodiment, discussion will be givenparticularly with respect to fatty acid ester. Fatty acid ester hasbetter dispersion property in varnish condition before baking of thecoating in comparison with low molecular weight polyethylene, and haslower melting point than amide imide to be precipitated on the surfaceof the coating. Furthermore, fatty acid ester contains non-polarhydrocarbon component (CH₂CH₂) to have no affinity with epoxy resin.

Therefore, bonding force between the surface of the primary coil andepoxy resin becomes small. A thickness of amide imide layer is in arange of 0.05 im to 5 im. No delamination effect cannot be attained atthe content of fatty acid ester less than 2 to 10% by weight with takingcontent of amide imide as 100% by weight, and if the content of fattyacid ester in excess of 10% by weight, heat resistance can be lowered.

When nylon or fluorinated material is used as a component not affinityor cause chemical coupling with the insulative resin, making process isincreased to result in cost up.

As set forth above, the reason why the component 5C having nocompatibility with epoxy resin 8 is to reduce the stress component 61(for satisfying the foregoing condition {circle around (1)} createdwithin the secondary bobbin by thermal shrinkage difference (linearexpansion coefficient difference) between the primary coil 5 and thesecondary bobbin 2 among stress 6 created within the secondary bobbin.

Also, it becomes possible to reduce delamination by thermal stressacting on the interface between the secondary coil and the secondarybobbin.

As set forth above, in the independent ignition type ignition coil whichis subject to severe temperature environment as installed within theplug hole, it becomes possible to reduce thermal stress of the secondarycoil due to linear expansion coefficient difference between componentmembers, ensure prevention of cracking in the secondary bobbin, andachieve high quality and high reliability of the ignition coil assemblyof this kind by maintaining good electrical insulation.

What is claimed is:
 1. An independent ignition type ignition coil for aninternal combustion engine to be used by directly connecting with eachignition plug of the internal combustion engine, in which a center core,a secondary coil wound around a secondary bobbin, and a primary coilwound around a primary bobbin are coaxially arranged in sequential orderfrom inside within a coil casing, and insulative resin being filledbetween these components, wherein said primary coil having a coatingfilm or coating layer containing non-polar hydrocarbon as componenthaving no affinity or not causing chemical reaction to an insulativeresin.
 2. An independent ignition type ignition coil for an internalcombustion engine to be used by directly connecting with each ignitionplug of the internal combustion engine, in which a center core, asecondary coil wound around a secondary bobbin, and a primary coil woundaround a primary bobbin are coaxially arranged in sequential order frominside within a coil casing, and insulative resin being filled betweenthese components, wherein said primary coil having a coating film orcoating layer containing CH₂CH₂n(n≧2) or CH₂—CH(CH₃)n(n≧2) ascomponent having no affinity or not causing chemical reaction to aninsulative resin.
 3. An independent ignition type ignition coil for aninternal combustion engine to be used by directly connecting with eachignition plug of the internal combustion engine, in which a center core,a secondary coil wound around a secondary bobbin, and a primary coilwound around a primary bobbin are coaxially arranged in sequential orderfrom inside within a coil casing, and insulative resin being filledbetween these components, wherein an outermost coating of said primarycoil having a coating film or coating layer containing any one of nylon,polyolefin such as polyethylene, polypropylene or the like, fluorinatedresin, fluorinated elastmer, fluorinated rubber, wax, fatty acid esteras component having no affinity or not causing chemical reaction to aninsulative resin.
 4. An ignition coil for an internal combustion engineas set forth in claim 1, wherein said secondary bobbin is PPS ormodified PPE.
 5. An independent ignition type ignition coil for aninternal combustion engine to be used by directly connecting with eachignition plug of the internal combustion engine, in which a center core,a secondary coil wound around a secondary bobbin, and a primary coilwound around a primary bobbin are coaxially arranged in sequential orderfrom inside within a coil casing, and insulative resin being filledbetween these components, wherein said secondary bobbin being for med ofPPS material, said primary bobbin being formed of PPS material, an innerinsulating coating on a surface of wire of said primary coil beingformed with a first coating taking ester imide as primary component anda second coating on outside of said first coating and taking amide imideas primary component, said second coating containing non-polarhydrocarbon having lower melting point than amide imide, and epoxy resinbeing filled between wire of said primary coil.
 6. An independentignition type ignition coil for an internal combustion engine to be usedby directly connecting with each ignition plug of the internalcombustion engine, in which a center core, a secondary coil wound arounda secondary bobbin, and a primary coil wound around a primary bobbin arecoaxially arranged in sequential order from inside within a coil casing,and insulative resin being filled between these components, wherein saidsecondary bobbin being formed of modified PPE material, said primarybobbin being formed of PBT material, an inner insulating coating on asurface of wire of said primary coil being formed with a first coatingtaking ester imide as primary component and a second coating on outsideof said first coating and taking amide imide as primary component, saidsecond coating containing non-polar hydrocarbon having lower meltingpoint than amide imide, and epoxy resin being filled between wire ofsaid primary coil.
 7. An ignition coil as set forth in claim 2, whereinsaid secondary bobbin is PPS or modified PPE.
 8. An ignition coil as setforth in claim 3, wherein said secondary bobbin is PPS or modified PPE.